We propose to investigate mating type in yeast in three distinct but interrelated areas. First, we plan to explore the mechanism by which heterochromatin is deposited and maintained using a novel visual assay for silencing at the mating type loci to determine the dynamics of interconversion between the active and repressed states and to assess the factors that influences those rates of conversion. These results should shed light on the regulatory processes affecting such diverse events as X-inactivation and chromosome imprinting, which underlies various diseases such as sporadic cancers. Second, donor selection during mating type interconversion requires developmentally-regulated, selective interaction between distant regions of the yeast genome. We plan to explore the mechanistic basis of this process by completing a genome-based screen for mutants altered in the selection process and by examining the dynamics of mating type interconversion in living cells to test the genetic underpinnings of this recombination process. These observations should yield insights into similar developmental events in larger eukaryotes, such as immunoglobulin class switching and sequential Pglobin activation. Finally, yeast cells respond to pheromone by initiating a transcriptional and developmental program culminating in mating. We recently found that individual cells within the population respond to pheromone in one of two distinct patterns: some cells show no response at all while the remaining cells exhibiting a full mating response. This bifurcated response is due in part to inhibitory cross talk among different MAP kinase signaling pathways. We propose to extend these studies to explore the crosstalk between the MAP kinase pathways responsible for pheromone response and for pseudohyphal and invasive growth. The results of these studies should shed light on the mechanism for signal integration in information processing in cells. All these studies, while probing different aspects of cell type and mating response in yeast, are unified by the common technical and theoretical approaches of examining individual cells and extracting biological and mechanistic principles from their distinct individual behaviors. [unreadable] [unreadable]